Hard disc drives are used in modern computer systems to enable users to store and retrieve vast amounts of data in a fast and efficient manner.
In a typical disc drive, one or more magnetic discs are rotated at a constant high speed and accessed by a rotary actuator assembly having a plurality of read/write heads that fly adjacent the surfaces of the discs on air bearings established by air currents set up by the rotation of the discs. Each head includes a write element that selectively magnetizes data fields defined on tracks on the corresponding disc surface during a write operation, and a read element that detects the selective magnetization of the data fields during a read operation. A read/write channel and an interface circuit, responsive to the heads, are provided to transfer the data between the discs and a host computer in which the disc drive is mounted.
A closed loop digital servo system is used to control the position of the heads relative to the tracks through the application of current to a coil of a voice coil motor. The tracks are defined from servo information written to servo fields on the surfaces of the discs during manufacturing. The data fields are subsequently arranged between adjacent servo fields during a disc drive formatting operation. The servo information typically includes gain control, synchronization, track address, radial position (index) and position information stored in associated fields. Thus, during operation of the disc drive the servo information is periodically sampled from the servo fields and used to control the position of the heads with respect to the tracks.
The frequency at which data are written to the data fields is selected to be as high as practicable in order to maximize data transfer characteristics of the disc drive. The use of magneto-resistive (MR) heads and partial response, maximum likelihood (PRML) read channel detection techniques have allowed disc drives of the present generation to write and read data at frequencies of up to 200 megahertz (MHZ). Typically, however, the servo information is written at a substantially lower frequency, such 20 MHZ. This reduction in the frequency at which servo information is written is due to a variety of considerations, including the fact that the retrieval of data is accomplished primarily through detecting the presence (or absence) of flux transition pulses from the media; compensation for the effects of factors such as noise and intersymbol interference can hence be sufficiently employed to decode the data, even when the pulses are adjacently disposed. However, proper operation of the servo system requires accurate determination of pulse location and amplitude of the servo information, which is typically difficult to accurately detect in the presence of noise and interference characteristic of information written at the higher frequencies used to store and retrieve the user data.
An MR head incorporates separate write and read elements, the write element comprising an inductive coil about a core with a write gap and the read element comprising a magneto-resistive material having a changed electrical resistance in the presence of a magnetic fields of a predetermined orientation. The increased sensitivity of the MR element allows write pulses from the head to be relatively narrow, facilitating the higher data transfer rates discussed above. The width of the write pulses is primarily a characteristic of the construction of the head so that the low frequency servo information is written with write pulses of the same width as the pulses used to write the data. Due to the lower write frequency, though, servo readback signals generally include pulses that are separated by baseline portions that should ideally have a nominal amplitude of about zero volts (or otherwise a value substantially equal to the reference level being used).
A pervasive problem often associated with the decoding of servo readback signals is baseline shift in such signals. Baseline shift, or shouldering, is characterized by a positive or negative shift in the amplitude of a baseline portion of the signal between adjacent peaks so that the readback signal takes non-zero values at times when a corresponding ideal signal should be close to zero.
Baseline shift is particularly troublesome when present in readback signals obtained from track address fields, as such baseline shift can interfere with the proper identification of the associated track address. It is common to encode track address information in the form of Gray code (GC), which generally comprises a multi-bit sequence, with each value in the sequence corresponding to a physical track address. Unlike a typical binary sequence, however, only one bit in the GC sequence changes as the head moves to each adjacent track, so as to facilitate improved track address detection. Each track is assigned a unique GC value and the total number of GC bits in each value is selected to accommodate the total number of tracks on each disc.
When a head reads a selected track address field (also referred to as a "Gray code" or "GC" field), an analog GC readback signal is obtained which is compared to both positive and negative detection thresholds and digitized into either ones or zeros, depending upon whether the signal is within or beyond the thresholds. A servo demodulator checks the resulting digitized value against the value that the GC field should generate to ensure the head is over the intended track. Thus, when baseline shift in the GC readback signal is sufficiently pronounced, the baseline portions can rise above or below the positive and negative detection thresholds, respectively, resulting in the baseline portion being erroneously detected as a peak.
Although the causes of baseline shift can vary and are not always clearly identifiable, it has been observed that factors such as the relative skew of the head with respect to the track (which varies with disc radius), electrical offsets and mechanical connections in the signal paths between the head and the servo circuit can provide significant contributions to the presence of baseline shift in GC readback signals. Moreover, it has been found that the use of high pass filtering in the preamp circuitry of a disc drive can increase the severity of baseline shift in GC readback signals, the high pass filtering stages utilized to reduce the effects of thermal asperities in the data readback signals.
As track and data bit densities and data transfer rates continue to increase with successive generations of disc drives, it is expected that such factors will generally lead to increases in the severity of baseline shift in servo readback signals. That is, improvements in read channel performance, continually demanded by the marketplace, should be implemented in a manner that does not degrade the performance of the servo circuit, which is often difficult to achieve. Accordingly, it is to the facilitation of continued increases in read channel data transfer rates while maintaining adequate levels of servo circuit performance that the present invention is directed.